During lactation there is a loss of bone mineral, regardless of the amount of calcium in the diet. However, severe and rapid loss of up to 50% of the bone occurs when lactating rats are placed on a severely calcium deficient diet. This occurs because of the increased demand for calcium in milk production. The lactating rat because of this demineralization is an excellent in vivo model for studying the effects of severe changes in mineralization. The effect of severe bone resorption on the mobilization of skeletal fluoride has not been studied. Fluoride incorporated into bone will be released as the bone mineral is resorbed, but the degree to which the released fluoride is redeposited is unknown. However, if a substantial amount of the released fluoride is redeposited, a 30-50% rapid reduction in bone mass should result in a significant increase in the fluoride concentration in the remaining bone mineral.